WifiNetworkConfigBuilder: Split into 2 separate builders

Split the existing WifiNetworkConfigBuilder into 2 separate builders,
one for building WifiNetworkSuggestion and one for WifiNetworkSpecifier.

Other changes in the CL:
a) Marked the constructor of the abstract NetworkSpecifier class public
to allow WifiNetworkSpecifier.Builder() (& hence WifiNetworkSpecifier) to
be public. The alternative is to put the builder for WifiNetworkSpecifier
in some other file (like WifiManager)
b) Mark WifiNetworkSpecifier class public to make it's builder visible.
c) Modify the documentation (& usage example) for WifiNetworkSpecifier
builder to explicitly call out that this can only be used for a local
wifi connection (i.e no internet capability).

Bug: 123313291
Test: ./frameworks/base/wifi/tests/runtests.sh
Test: make docs
Change-Id: I1a5fcff886124010222e112dbca92797a514b6d9